Introduce IERS_A_URL_MIRROR for when IERS_A_URL is down

[pllim]

This PR introduces IERS_A_URL_MIRROR for when IERS_A_URL is down. Fix #8304

Local testing with -P coordinates --remote-data and -P utils --remote-data worked for me. ™️ I also checked that IERS_A_URL` was still down when those tests ran.

Note: Check numpy-dev job (set to be allowed to fail) and make sure it is green before merge.

[mhvk]

This looks good, thanks for the quick implementation! Only one total nitpick (feel free to ignore and certainly avoid testing!)

[codecov]

Codecov Report

Merging #8308 into master will increase coverage by <.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #8308      +/-   ##
==========================================
+ Coverage   86.73%   86.74%   +<.01%     
==========================================
  Files         385      385              
  Lines       58009    58028      +19     
  Branches     1060     1060              
==========================================
+ Hits        50317    50334      +17     
- Misses       7077     7079       +2     
  Partials      615      615

Impacted Files	Coverage Δ
astropy/utils/iers/iers.py	92.16% <92.85%> (+0.19%)	⬆️
astropy/samp/hub.py	77.38% <0%> (-0.14%)	⬇️

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[mhvk]

Damn, I forgot my [skip ci]. Well, I guess at least we can see if circle ci now passes (it should).

[mhvk]

OK with the final docstring change

[mhvk]

OK, tests passed so merging!

A query mostly out of curiosity: I wondered why the travis tests just hung rather than show a time-out - is this because subsequent tests kept trying to download the file?

[pllim]

why the travis tests just hung rather than show a time-out

That I am not sure. Maybe it is stuck when one of the imports somehow try to download the file at test collection? 🤷‍♀️ (Just a blind guess.)